The present invention relates to a power steering system for a vehicle, especially an industrial vehicle such as a heavy truck.
A power steering system has been a standard feature in vehicles for many years, in particular in industrial vehicles. Such a system assists the vehicle driver by applying a force to the wheels, through a steering actuator, which complements the effort of the driver which is mechanically transmitted from the steering wheel to the steered wheels of the vehicle. Therefore, a lower effort is required for the driver when turning the steering wheel and the vehicle is easier to manoeuvre.
Typically, a power steering system comprises a steering gear including the actuator capable of providing a steering assistance effort to at least one of the wheels of the axle. This actuator is controlled by a parameter related to the steering wheel movement, for example as a function of the steering effort exerted by the driver on the steering wheel.
A mechanical arrangement makes it possible, on the one hand, to transmit the movement of the actuator to the steered wheel and, on the other hand, to transmit the movement of said steered wheel to the other wheel of the same axle.
However, a significant drawback of such a conventional power steering system is that it comprises heavy pans, and that it requires space—while the corresponding allotted space is generally limited. Therefore, the implementation of such a system leads to heavy packaging work and involves some limitations due to the size of the wheels and/or of the vehicle frame.
It therefore appears that, from several standpoints, there is room for improvement in power steering systems for vehicles.
It is desirable to provide an improved power steering system for a vehicle.
It is also desirable to provide a power steering system which requires less space than the prior art systems and which has a reduced weight, while also being effective and reliable.
According to a first aspect, the invention relates to a power steering system for a vehicle having at least one steered axle joining two wheels, the power steering system comprising a first steering mechanism designed to control the steering of at least one of the wheels, and comprising:                a mechanical transmission of the movement from a steering wheel of the vehicle to at least one of the wheels;        a first actuator controlled by a parameter related to the steering wheel movement and capable of providing a steering assistance effort to at least one of the wheels;        
the power steering system further comprising a second steering mechanism designed to control the steering of at least one of the wheels of said same axle, said second steering mechanism including a second actuator capable of providing a steering assistance effort to at least one of the wheels of said axle, the second actuator having:                a housing which is secured to the axle;        
a first and a second piston rods able to slide through the housing and extending, on opposite sides, from inside to outside of the housing, with said piston rods being substantially parallel to said axle.
Thus, with the power steering system according to an aspect of the invention, the wheels of the axle can be steered by means of a first actuator and/or a second actuator. In practice, the steering assistance is primarily provided, by the first actuator, while the second actuator can either act simultaneously with the first one, to provide an additional steering assistance to the wheels of the same axle, or not be used, depending on the current needs.
More precisely, an advantageous way of using an aspect of the invention is to provide a first actuator which is dimensioned in order to be capable to provide the steering assistance required in most cases, for example in 90% of cases. The second actuator is then used to provide the extra steering assistance needed in specific conditions, preferably occasionally. The second actuator therefore acts as a means intended to provide a boost to steering assistance when imposed by conditions. For example, the first actuator can be dimensioned to meet the needs for a vehicle running at high speed on a highway, and the second actuator can be used to provide the additional effort which is required for manoeuvring the vehicle at low speed.
As a consequence, owing to an aspect of the invention, the first actuator size can be reduced as compared to conventional actuators, without waiting the power steering system efficiency insofar as a second actuator is provided for specific cases. This results in a less cumbersome steering gear, therefore saving space in the area of said steering gear and making it possible to lower the cab and/or implement a bigger cooling pack, for example.
Downsizing the first actuator also results in downsizing the mechanical arrangement connecting the steering gear and the steered wheel, in particular the drag link. Components of the first steering mechanism are thus less cumbersome and less heavy, which is a further advantage of an aspect of the invention.
Besides, the impact of the second actuator on the system overall size is fairly low. On the one hand, there is no need to install said second actuator in the steering gear or close to it. Thus, said actuator can be located in an area where more space is available. On the other hand, positioning the piston rods of the second actuator parallel to the axle makes it possible to reduce the space occupied by said second actuator, and to avoid protruding parts.
Even is the second actuator according to an aspect of the invention can be an actuator of any type and, for instance, an electric or a pneumatic actuator, the second actuator is preferably a hydraulic actuator. In this case the housing of the second actuator is a cylinder barrel and the second actuator comprises a movable piston located in said cylinder barrel and defining inside it at least a first and a second chambers. The piston is connected, on opposite sides thereof, respectively to said first and second piston rods. Thanks to this hydraulic arrangement, the packaging of the second actuator and so the packaging of the power steering system according to an aspect of the invention is greatly improved.
According to an other preferred embodiment, the first steering mechanism comprises a track rod connected to the wheels of said axle in order to allow transmission of the steering movement from one wheel to the other wheel, the piston rods forming part of the track rod.
In other words, the axis of the second actuator is coincident with the track rod axis, the cylinder barrel being interposed between two portions of the track rod. Typically, the track rod can include two links rods and the two piston rods, each link rod having an end connected to the end of a piston rod opposite the piston of the second actuator and a second end connected to one wheel, possibly by means of an intermediate member.
This disposition makes it possible to further reduce the size of the power steering system, more specifically in the area of the second actuator. Furthermore, with this implementation, the second actuator can steer both wheels, and higher efforts can be transmitted to the wheels by the second actuator.
In an implementation, the axle is a front axle of the vehicle, either the single front axle of the vehicle or one of the front axles of the vehicle.
The first actuator can be an electric, actuator, which is particularly interesting especially for long haul trucks that need a fairly low steering assistance when running on highways, since it has a low consumption.
Alternatively, the first actuator can be a hydraulic, actuator, which is capable of providing a high steering assistance.
In this embodiment, the second actuator can be hydraulically coupled to the first actuator, so that the pressures in the chambers of the second actuator are substantially identical to the pressures in the chambers of the first actuator, when said actuators are connected.
The power steering system may further comprise disconnecting means arranged between the first and second actuators in order to allow the second actuator to be hydraulically disconnected from the first actuator. Disconnecting the actuators may be done in case of a failure, especially on the second actuator or on the corresponding hydraulic circuit, the disconnecting means then having a safety function. The actuators may also be disconnected if no additional steering assistance is needed, the first actuator being sufficient to provide alone the required assistance; in this case, the piston of the second actuator can move freely inside the cylinder barrel, in order not to impair the operation of the first steering mechanism.
Preferably, the first actuator can be always active, meaning that it is permanently operatively connected to one wheel, whether it actually provides steering, assistance or not at a given moment. On the contrary, the second actuator can be coupled to the first actuator, meaning that there exist structural means effectively providing a possible connection between said actuators, but either connected or disconnected from said first actuator, meaning, that, functionally, there is actually or not a fluid communication between the actuators.
Providing disconnecting means can be particularly advantageous in case the second actuator is located on a front axle, insofar as it is then exposed and could be damaged.
For example, the disconnecting means comprise an isolation valve which can be switched between:                a first position, in which the second actuator is in fluid communication with the first actuator;        and a second position, in which the second actuator is hydraulically isolated from the first actuator and the first and second chambers of the second actuator are in fluid communication.        
This isolation valve can typically be arranged on conduits joining the chambers of the first and the second actuators, and can be controlled by control means depending on various parameters as will be explained later. The first position can be used during normal operation of the system, when there is a need for an extra steering assistance. The second position can be used either during normal operation of the system, when no extra steering assistance is required, or in case of a failure, especially on the second actuator or its hydraulic circuit. In the second position, the first actuator can steer wheels independently from the second actuator.
The power steering system can comprise control means capable of switching the isolation valve from one position to another depending on the pressure received by the first actuator. For example, an increase in said pressure indicates that there is a need for further steering assistance, and as a consequence, the second actuator is activated, the isolation valve being moved to its first position.
The power steering system can further comprise a pump arranged to provide the extra flow of hydraulic fluid required in case the second actuator is hydraulically connected to the first actuator (i.e. when the isolation valve is in its first position). This pump can be an additional pump, such as an electro pump. Alternatively, the power steering system can comprise a variable displacement pump capable of adjusting the flow it provides, for example a pump with a first low displacement and a second high displacement.
The second actuator can be controlled by a parameter related to the steering wheel movement, such as the steering wheel angle, the steering wheel speed, or the steering effort—or torque—on the steering wheel. In case an isolation valve is provided, said control means can be capable of switching the isolation valve from one position to another depending on said parameter.
Alternatively, the second actuator can be controlled by the pressure received by the first actuator.
Besides, the power steering system can comprise control means capable of deactivating the second actuator, in particular in case the vehicle speed is greater than a predetermined threshold, for example above 60 km/h. Indeed, at relatively high speeds, there is no need for a high steering assistance effort. In case an isolation valve is provided, said control means can be capable of switching the isolation valve to the second position when the vehicle speed is greater than said threshold.
According to a second aspect, the invention relates to a vehicle having at least one steered axle joining two wheels and comprising a power steering system as previously described.
These and other features and advantages will become apparent upon reading the following description in view of the drawing attached hereto representing, as non-limiting examples, embodiments of a vehicle according to an aspect of the invention.